Real-time fMRI neurofeedback: progress and challenges.

In February of 2012, the first international conference on real time functional magnetic resonance imaging (rtfMRI) neurofeedback was held at the Swiss Federal Institute of Technology Zurich (ETHZ), Switzerland. This review summarizes progress in the field, introduces current debates, elucidates open questions, and offers viewpoints derived from the conference. The review offers perspectives on study design, scientific and clinical applications, rtfMRI learning mechanisms and future outlook.

Neural representation and the cortical code.

The principle function of the central nervous system is to represent and transform information and thereby mediate appropriate decisions and behaviors. The cerebral cortex is one of the primary seats of the internal representations maintained and used in perception, memory, decision making, motor control, and subjective experience, but the basic coding scheme by which this information is carried and transformed by neurons is not yet fully understood. This article defines and reviews how information is represented in the firing rates and temporal patterns of populations of cortical neurons, with a particular emphasis on how this information mediates behavior and experience.

A multielectrode implant device for the cerebral cortex.

A new class of brain implant technology was developed that allows the simultaneous recording of voltage signals from many individual neurons in the cerebral cortex during cognitive tasks. The device allows recording from 49 independent positions spanning a 2 x 2-mm region of neural tissue. The recording electrodes are positioned in a square grid with 350 microm spacing, and each microelectrode can be precisely independently vertically positioned using a hydraulic microdrive. The device utilizes ultrafine, sharp iridium microelectrodes that minimize mechanical disturbance of the region near the electrode tip and produce low noise neuronal recordings. The total weight of this device is less than 20 g, and the device is reusable. The implant device has been used for transdural recordings in primary somatosensory and auditory cortices of marmosets, owl monkeys, and rats. On a typical day, one-third of the microelectrodes yield well-discriminated single neuron action potential waveforms. Additional array electrodes yield lower amplitude driven multiunit activity. The average signal-to-noise ratio of discriminated action potential waveforms 6 months after implantation was greater than 9. Simple design alternatives are discussed that can increase the number of electrodes in the array and the depths at which dense array recordings can be achieved.

Optimizing sound features for cortical neurons.

The brain's cerebral cortex decomposes visual images into information about oriented edges, direction and velocity information, and color. How does the cortex decompose perceived sounds? A reverse correlation technique demonstrates that neurons in the primary auditory cortex of the awake primate have complex patterns of sound-feature selectivity that indicate sensitivity to stimulus edges in frequency or in time, stimulus transitions in frequency or intensity, and feature conjunctions. This allows the creation of classes of stimuli matched to the processing characteristics of auditory cortical neurons. Stimuli designed for a particular neuron's preferred feature pattern can drive that neuron with higher sustained firing rates than have typically been recorded with simple stimuli. These data suggest that the cortex decomposes an auditory scene into component parts using a feature-processing system reminiscent of that used for the cortical decomposition of visual images.

Primary cortical representation of sounds by the coordination of action-potential timing.

Cortical population coding could in principle rely on either the mean rate of neuronal action potentials, or the relative timing of action potentials, or both. When a single sensory stimulus drives many neurons to fire at elevated rates, the spikes of these neurons become tightly synchronized, which could be involved in 'binding' together individual firing-rate feature representations into a unified object percept. Here we demonstrate that the relative timing of cortical action potentials can signal stimulus features themselves, a function even more basic than feature grouping. Populations of neurons in the primary auditory cortex can coordinate the relative timing of their action potentials such that spikes occur closer together in time during continuous stimuli. In this way cortical neurons can signal stimuli even when their firing rates do not change. Population coding based on relative spike timing can systemically signal stimulus features, it is topographically mapped, and it follows the stimulus time course even where mean firing rate does not.

Inputs from the ipsilateral and contralateral vestibular apparatus to behaviorally characterized abducens neurons in rhesus monkeys.

1. We made extracellular recordings from neurons in the abducens nuclei of alert rhesus monkeys during electrical stimulation of the vestibular labyrinths with brief current pulses and during smooth pursuit, steady fixation, and the vestibuloocular reflex (VOR) evoked by passive head turns. The responses to electrical stimuli were compared with quantitative measures of the sensitivity of each neuron to eye position and eye velocity. We also compared the strengths of the vestibular inputs from the labyrinths ipsilateral and contralateral to the side of recording. 2. Abducens neurons showed transient excitation after a current pulse was applied to the contralateral labyrinth and transient inhibition after stimulation of the ipsilateral labyrinth. The latency of excitation had a mean value of 1.7 ms and a median value of 1.5 ms. Latency was unimodally distributed with little variation among neurons. Neurons with large responses showed a second phase of excitation that started 2.5 ms after the stimulus. 3. In two of three monkeys, the excitatory responses of abducens neurons to electrical stimulation of the contralateral labyrinth were approximately 3 times as large as their inhibitory responses to stimulation of the ipsilateral labyrinth. The difference in response size was not observed in the third monkey. The asymmetry in the size of the electrically evoked inputs from the two labyrinths was associated with a smaller asymmetry in responses of abducens neurons during the VOR evoked by passive head turns. The increase in firing rate during head rotation away from the side of the recording was almost always larger than the decrease in firing rate during head rotation toward the side of the recording. 4. The size of the neuronal response to electrical stimulation was correlated with the magnitude of the change in discharge rate during eye movements. Single or multiple regression of measures of response amplitude against eye position threshold, sensitivity to eye position, sensitivity to eye velocity, and baseline discharge rate yielded correlation coefficients that ranged from 0.26 to 0.92 in different monkeys. The existence of positive correlations is consistent with a role of the intrinsic properties of abducens neurons in determining recruitment order. However, the existence of large amounts of variability within most of the samples suggests that the recruitment order of abducens neurons also depends on the discharge properties of the afferents to each abducens neuron.